In 2007, there were 1.8 million alternative fuel vehicles sold in the United States, indicating an increasing popularity of alternative fuels. There is growing perceived economic and political need for the development of alternative fuel sources due to general environmental, economic, and geopolitical concerns of sustainability.
The major environmental concern is that most of the observed increase in globally averaged temperatures since the mid-20th century is due to the observed increase in greenhouse gas concentrations. Since burning fossil fuels is known to increase greenhouse gas concentrations in the atmosphere, it is believed that they are a likely contributor to global warming.
Although fossil fuels have become the dominant energy resource for the modern world, alcohol has been used as a fuel throughout history. The first four aliphatic alcohols (methanol, ethanol, propanol, and butanol) are of interest as fuels because they can be synthesized and they have characteristics which allow them to be used in current engines. Biobutanol has an energy density that is closer to gasoline than the other alcohols; however, this advantage is outweighed by disadvantages (compared to ethanol and methanol) concerning, for example, production.